1. Field of the Invention
The present invention relates to an exhaust gas recirculation system and more particularly to a control system for same.
2. Description of the Prior Art
As is well known exhaust gas recirculation (EGR) suppresses the formation of NOx (i.e. the various oxides of nitrogen formed during high temperature pressure combustion) within the combustion chambers of internal combustion engines due to reduction of the combustion rate and according reduction of peak combustion temperatures. As is also well known the amount of EGR must be carefully proportioned with respect to the volume of air inducted into the engine so as to form an air/EGR gas/fuel mixture which exhibits the desired rate of combustion and reduction of peak combustion temperatures. In order to control the amount of EGR gas recirculated and thus form the appropriate air/EGR gas fuel mixture during all modes of engine operation various control systems have beem proposed. Many of these systems use the induction manifold vacuum as a source of motive power and feed same to a vacuum motor operatively connected to a valve which controls the actual flow of exhaust gases from the exhaust system to the induction system. To control the operation of the vacuum motor a pressure regulating device sensitive to one or more operating parameters of the engine modulates the degree of vacuum prevailing in the vacuum chamber of the vacuum motor by introducing atmospheric air into the chamber and/or conduiting connected thereto.
However, a drawback has been encountered with such systems in that insufficient vacuum prevails within the induction manifold at high load and low engine speed operation of the engine so that insufficient force is generated by the vacuum motor to open the EGR valve (as it will be referred to hereinafter) and accordingly insufficient exhaust gas recirculation takes place.
To overcome the above described drawback, it has been proposed to reduce the biasing force of the spring housed in the vacuum chamber so that the vacuum available under high load low RPM conditions opens the EGR valve sufficiently and an adequate supply of exhaust gas is recirculated to the induction system of the engine. However, another drawback has been encountered when using a spring of the type described above and that is the maximum vacuum or minimum absolute pressure permitted to prevail in the vacuum chamber must be limited, viz., the range between the absolute pressure which permits the EGR valve to close and that which opens it completely is considerably restricted by the provision of the less powerful spring. Furthermore this restricted range necessitates very careful control of the modulation of the vacuum to prevent the EGR valve snapping from an open position to a closed position or vice versa. In practice however the vacuum motor equipped with the aforementioned less powerful spring is overly sensitive to small variations in the vacuum fed thereinto from the induction system and the afore described digital or on/off action wherein the valve snaps from one extreme position to another in fact often occurs during normal operation of the vehicle in which the engine is disposed. This on/off action naturally causes erratic engine operation, the supply of exhaust gases being suddenly permitted or cut off, whereby engine performance and emission control deteriorate markedly.
Furthermore the erratic engine operation can under certain conditions be such that the drive is unwantedly distracted by the jolting and surging of the vehicle to a point where he or she is unable to safely control same.
Thus there still remains a need for an EGR control system which recirculates adequate amounts of exhaust gas during high load low RPM operations, eliminates the on/off action of the prior art replacing same with smooth and continuous movement between open and closed positions, while still providing adequate control of the amount of exhaust gas recirculating during other modes of engine operation. This of course includes reducing the rate of exhaust gas recirculating at high speed low load operation during which the production of NOx is inherently low and the normal rate of recirculation provides an excessive amount of exhaust gas.